1. Field of the Invention
This invention relates generally to so-called "lost foam" methods for casting metals. More specifically, it concerns methods for preparing various novel specifically defined heat-destructible shaped-foam patterns for use in replica-casting of metals (particularly low carbon steels) by the lost foam technique (particularly techniques involving "top gating"). It also concerns various novel expandable and expanded plastic materials.
Lost foam casting essentially involves pouring molten metal into a pattern having a heat-destructible portion of a cellular plastic material (or foam), while the pattern and its entry port(s), or "gate(s)", are essentially surrounded and supported by highly compacted refractory material such as sand.
2. Description of the Related Art
In the past, commercial processes have mainly involved the use of foam patterns in which the plastic material was polystyrene. However, there are problems with use of expandable polystyrene (EPS) in lost foam casting, also called evaporative pattern casting, where the pattern or core assembly is partially or wholly EPS.
One problem is that carbonaceous nonvolatile EPS residue floats on molten iron and becomes trapped inside the cavity formed by the decomposing polymeric foam. The large amount of residue results in carbon-containing voids, called carbon defects, weak points and leaks through the casting. This leads to inefficient manufacturing and component failures.
A second problem with EPS molded patterns or core assemblies is that of shrinkage. An EPS molded part with a hydrocarbon blowing agent, such as pentane, loses most of the blowing agent in a period of one month or less at room temperature. Simultaneous with the loss of blowing agent, shrinkage of the molded parts occurs. This dimensional change is undesirable, especially if molded parts are to be stored for an extended period or if molded parts are to be cast during the period while shrinkage is occurring, especially if the tolerance of the cast part is critical.
Recently published Japanese Patent Disclosure Kokai No. 60-18,447 has working examples concerning the use of foam patterns prepared from polystyrene or several copolymers derived from raw materials including methyl methacrylate and alpha-methyl styrene, in casting iron and aluminum by the "bottom gate" casting technique. It also has broader general teachings. For example, it proposes that the lost foam substrate can be a homopolymer of methyl methacrylate, and that the molten metal may also be zinc, brass, or steel.
Prior art methods of lost foam casting have now been found to be inadequate and unable to prepare superior metal castings for many types of metal (such as steels having a very low carbon content) and/or many types of casting techniques (such as "top gate" techniques involving the use of downwards flow of the molten metal into the heat destructible pattern, rather than merely "bottom gate" techniques involving upwards movement of the molten metal).